1. Field of the Invention
The present invention relates to an image forming apparatus, an image forming method, and a computer program product.
2. Description of the Related Art
In recent years, in the field of an electrophotographic color image forming apparatus, there has been proposed an image forming apparatus that uses both a direct transfer method for directly transferring an image formed on a photosensitive element onto a sheet and an indirect transfer method for temporarily transferring images formed on a plurality of photosensitive elements for each color onto an intermediate transfer member so as to superimpose the images one on top of the other and then transfer the superimposed images onto a sheet (see, for example, Japanese Patent Application Laid-open No. 2008-90092).
More specifically, Japanese Patent Application Laid-open No. 2008-90092 discloses a technology in which, as a method of performing alignment between a directly-transferred image and an indirectly-transferred image in the combination-type image forming apparatus as mentioned above, a time required for moving a belt from a primary transfer position, at which images on a plurality of photosensitive elements for each color are transferred onto an intermediate transfer belt, to a direct transfer position is set to be an integral multiple of one rotation cycle of a drive roller that rotates the intermediate transfer belt, whereby misalignment of the transferred images due to the fluctuation of the rotation velocity of the drive roller is minimized.
However, in the technology disclosed in Japanese Patent Application Laid-open No. 2008-90092, consideration is only given to the velocity fluctuation of the intermediate transfer belt, not to the velocity fluctuation of a transfer-sheet conveying belt. Therefore, there is a problem in that it is difficult to improve position accuracy for alignment at the time of performing full-color printing by using both the direct transfer system and the indirect transfer system.